Photophoretic Levitation and Trapping of Dust in the Inner Regions of Protoplanetary Disks

TL;DR

This paper investigates how photophoresis influences dust grain distribution in protoplanetary disks, revealing a new dust trapping mechanism that could impact planet formation processes.

Contribution

It introduces a novel, consistent treatment of photophoresis forces in radiative transfer models, demonstrating their significant role in dust levitation and trapping in disk inner regions.

Findings

Large dust grains are trapped several scale heights above the mid-plane.

Photophoretic trapping can increase dust-to-gas ratio by up to 100 times.

Dust trapping depends strongly on particle size and porosity.

Abstract

In protoplanetary disks, the differential gravity-driven settling of dust grains with respect to gas and with respect to grains of varying sizes determines the observability of grains, and sets the conditions for grain growth and eventually planet formation. In this work we explore the effect of photophoresis on the settling of large dust grains in the inner regions of actively accreting protoplanetary disks. Photophoretic forces on dust grains result from the collision of gas molecules with differentially heated grains. We undertake one dimensional dust settling calculations to determine the equilibrium vertical distribution of dust grains in each column of the disk. In the process we introduce a new treatment of the photophoresis force which is consistent at all optical depths with the representation of the radiative intensity field in a two-stream radiative transfer approximation. The levitation of large dust grains creates a photophoretic dust trap several scale heights above the mid-plane in the inner regions of the disk where the dissipation of accretion energy is significant. We find that differential settling of dust grains is radically altered in these regions of the disk, with large dust grains trapped in a layer below the stellar irradiation surface in where the dust to gas mass ratio can be enhanced by a factor of a hundred for the relevant particles. The photophoretic trapping effect has a strong dependence on particle size and porosity.